Nucleonic gauges having source window protector plates with compressed air wipe arrangements for use in oil spray environments

ABSTRACT

A nucleonic gauge for use with moving strip or web material in which oil particles or oil mist is entrained with the web. The nucleonic gauge incorporates protector plates, there being one plate associated with the source housing and another plate associated with the detector housing. Each plate includes air passageways through which air is directed under pressure at a region closely adjacent the source window and the detector window for preventing the occurrence of oil droplets or other foreign material on these windows. The plates further provide mechanical protection against window damage in the event that there is a break in the sheet material.

United States Patent McMullen et a].

May 9, 1972 [54] NUCLEONIC GAUGES HAVING SOURCE WINDOW PROTECTOR PLATESWITH COMPRESSED AIR WIPE ARRANGEMENTS FOR USE IN OIL SPRAY ENVIRONMENTSPrimary Examiner-James W. Lawrence Assistant Examiner-Morton .I. F romeAtt0rneyMarechal, Biebel, French & Bugg A nucleonic gauge for use withmoving strip or web material in which oil particles or oil mist isentrained with the web. The nucleonic gauge incorporates protectorplates, there being one plate associated with the source housing andanother plate associated with the detector housing. Each plate includesair passageways through which air is directed under pressure at a regionclosely adjacent the source window and the detector window forpreventing the occurrence of oil droplets or other foreign material onthese windows, The plates further provide mechanical protection againstwindow damage in the event that there is a break in the sheet material.

ABSTRACT 9 Claims, 9 Drawing Figures [72] lnventors: James M. McMullen,Columbus; Douglas A. Berg, Worthington, both of Ohio [73) Assignee:Industrial Nucleonics Corporation [22] Filed: Dec. 15, I969 {2i} Appl.No.1 885,222

[52] U.S. Cl ..250/83.3 D, 250/435 D, 250/52 [Sl] Int. Cl. ..G0lt I/l6[58] FieldofSearch "250/833 D,43.5 D,50,52

[56] References Cited UNlTED STATES PATENTS 2,8l4,953 l2/l957 Callaghan..73/355 3,426,200 2/1969 Lehman et al, ..250/83.3 D

[Ill/ll 11/4 I i 26 l 40 I "ML 45 J 92 20 w' '11s 90 L 1 loo I02PATENTEMY SL972 3,662,174

SHEET 1 OF 2 FIG-1 AIR SUPPLY 83 1" INVENTORS JAMES M. MEMULLEN 8| 2ODOUGLAS A. BERG 5 I ATfOR/VEY? PATENTEDMM 9:912

SHEET 2 [IF 2 FIG-7 72 7O TTT FIG-9 N UCLEONIC GAUGES HAVING SOURCEWINDOW PROTECTOR PLATES WITH COMPRESSED AIR WIPE ARRANGEMENTS FOR USE INOIL SPRAY ENVIRONMENTS BACKGROUND OF THE INVENTION The present inventionrelates to the art of nucleonic gauging and more particularly to theon-line measurement of the thickness or basis weight of moving strip orsheet material. A particular problem which occurs in an industrialenvironment is that of the accumulation of foreign material on thesource and/or detector windows. Such accumulation causes errors in theread out since the foreign material results in absorption of thenucleonic energy and is read as a change or increase in basis weight orthickness. This problem is particularly acute in installations where thenucleonic gauge is used for measuring the thickness or weight of sheetaluminum or sheet steel as it passes from a rolling mill. In these andsimilar industrial environments the sheet is bathed in oil, and a mistor cloud of oil forms which is carried along by the sheet. The entrainedoil has a tendency to collect and form droplets on the surfaces of thewindows of the source and the detector housings. As the dropletsaccumulate, fall, and run they present an everchanging mass in the passgap which adversely affects the accuracy of the measurement.

A further difficulty of sensitive gauges is that of protecting thesource and detector windows against damage and breakage which is likelyto occur in the event that the sheet material breaks. The windows, ofnecessity, are formed of thin, light weight material. Sheet mylar is anexample. As a result these windows are more fragile than are the otherexposed parts of the gauge. The web or sheet material being measured ismoving at a relatively high velocity, which may be in the order of 500feet per minute or more, and the pile up of sheet material against thegauge, were the window not other wise protected, frequently results inthe window being torn or broken, thus necessitating a shut down of theline until the gauge can be repaired and placed back into service.

SUMMARY OF THE INVENTION The present invention is directed to apparatusfor protecting the source and detector windows and for decreasing thelikelihood of window damage in an industrial accident of the typereferred to above and, at the same time, for forming a means by whichthe windows may be kept free of the accumulation of dilatoriousmaterial, such as droplets of oil, dust, particles of metal, or othermaterial which may be in the region of the pass gap. For this purpose,the source and detector heads are provided with plate means which mayform a permanent part of the installation. This preferably includes afirst plate mounted on the detector head in the region of the pass gapand formed with an aperture which is aligned substantially with thedetector window. The aperture in this plate is chosen so as to be insubstantial non-interferring relation with the nucleonic energy, butpreferably is formed with a diameter which is somewhat smaller than thatof the window, to provide further mechanical protection for the window.The plate is further formed with an air passageway which opens into theregion of the plate aperture and is arranged so as to direct air at highvelocity into wiping impingement with and across the window so aseffectively to prevent the accumulation of oil droplets on the windowsurface.

A second plate, similar to the first plate, is provided for the sourcehousing and is similarly positioned in the pass gap and is provided withan opening or aperture arranged in substantial alignment with the sourcewindow. Again, this opening is chosen to have a diameter which is innon-interfering relation to the path of the nucleonic energy, but,preferably less than that of the source window. The resulting structureis one in which both the source and the detector windows are recessedinwardly of the surfaces of these plates so that the likelihood ofbreakage of the windows is substantially reduced. The second plate issimilarly formed with an air passageway opened into the plate aperturefor directing air under pressure across the source window to prevent theaccumulation of foreign material thereon. Preferably, the second platealso is provided with an air exit passage through which air andentrained material is removed from the measuring region.

As a further feature of the invention, the upper plate, which isnormally the plate associated with the detector, is formed with anannular chamfer at the opening defining an inclined wall along whichdroplets of oil may naturally flow out of the measuring region. As theoil collects, it drops at a location outside of the measuring region.This is particularly important in that the drops do not fall on thesource window when the gauge is off sheet for source standardization.

It is accordingly an important object of this invention to provideplates for mechanically protecting the windows of source and detectorhousings on a nucleonic gauge.

Another important object of this invention is the provision of an airwipe for the windows of a nucleonic gauge which effectively prevents theaccumulation of oil droplets and the like on the windows.

A further object of the invention is the provision, in a nucleonicgauge, of window protecting devices associated with the source anddetector housings which incorporate air passages through which air maybe directed at relatively high velocity for wiping the window surfacessubstantially free of the accumulation of oils and the like and forproviding mechanical protection for the windows.

These and other objects and advantages of the present invention will beapparent from the following description, the accompanying drawings andthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of agauge constructed according to this invention;

FIG. 2 is a fragmentary plan view of the source housing;

FIG. 3 is a side elevation of the source housing;

FIG. 4 is a front elevation of the source housing;

FIG. 5 is a fragmentary plan view looking upwardly at the detectorhousing;

FIG. 6 is a front view of the detector housing;

FIG. 7 is an enlarged vertical section through the source and detectorhousings taken generally along the reference lines 7-7 ofFIGS. 2 and 5;

FIG. 8 is a plan view of the inside surface of the plate on the detectorhousing looking along the lines 8 8 of FIG. 6; and

FIG. 9 is a fragmentary enlarged section taken generally along the line9-9 of FIG. 8.

DESCRIPTION OF PREFERRED EMBODMENT Referring to the figures of thedrawing, a nucleonic thickness measuring gauge constructed according tothis invention is shown generally at 10 in FIG. I as including atraversing C-bracket 12. The bracket 12 may be suitably mounted on atransversely oriented track, not shown, so that the bracket may be movedto an on-line measuring position as shown in FIG. I and to an off-lineposition, such movement being in the direction as identified by thearrow 13. An example of a suitable traversing arrangement is shown inI-Iolben U.S. Pat. No. 2,967,242 of l96l. The general structure andoperation of the gauge may be according to the teaching of Chope U.S.Pat. No. 2,790,945 of 1957 or Chope U.S. Pat. No. 2,829,268 of 1958, andmay be operated at a fixed transverse position with respect to the web Wof the moving strip or sheet material, or alternatively, the gauge maybe caused to take profile measurements by traversing movement, accordingto known practices in the art.

The C-bracket 12, at its outer extremities, forms the housings whichcontain the nucleonic radiation source and the detector. For thispurpose, the upper arm I4 of the bracket 12 is terminated at itsextremity in a rectangular structure defining a detector housing 15, thefurther details of which are shown in FIGS. 5, 6 and 7. The lower arm 18is similarly terminated in a rectangular structure defining a sourcehousing 20 which is positioned and spaced vertically beneath thedetector housing 15, thus forming and defining therebetween a pass gap21, as best shown in FIG. 7, through which the web W of sheet materialpasses during measurements. The details of the source housing 20 arefurther illustrated in H68. 2, 3, 4, and 7.

Referring first to the detector housing 15, it comprises essentiallyfour rectangularly arranged plates or sheets, comprising a removable topsheet 22, a pair of spaced parallel side sheets 23 and 24 (FIGv a bottomsheet 26, and an end sheet 27. These sheets, which may consist of /4sheet steel, together form the detector housing within which may becontained an ion chamber detector 28 which may be constructed accordingto the teachings of Martin et al. US. Pat. No. 3,396,275 of 1968.According to conventional practice, the lower or bottom sheet 26 of thedetector housing 15 is formed with a circular opening 30 in substantialalignment with the chamber 28, and this opening is closed by a thinsheet 32 of plastic film or other suitable material and defines thesource window 33. The sheet 32 is positioned on the upper surface of thelower plate 26 and is removably retained by a retainer ring 34 by accessthrough the removal of the cover sheet 22.

Referring to the source housing 20, this is similarly formed with anupper sheet 40, a pair of parallel disposed side sheets 42 and 43 (FIG.2) a bottom sheet 44, and an end plate 45, secured in a box orrectangular like construction. The parts forming the source housing mayalso be formed of A inch sheet steel.

The upper sheet 40 receives a source housing assembly indicatedgenerally at 46 in FIG. 7, and this assembly includes a source andshield 47, a movable shutter 48 and a source window 50. The window 50,like the window 33, is preferably formed of a sheet 52 of thin plasticmaterial and is retained on the assembly 46 by a retainer ring 54. Inthe case of the source housing, the window 50 is retained substantiallyflush with the upper surface of the plate 40 on the apertured sourceassembly 46 and defines the window through which radiation may pass fromthe source 47 to the detector 28. The nucleonic source 47 may beconstructed according to known practices to include a penetrativeradiation material by which a source of beta, gamma, brensstrahlung orother type of radiation is produced to erradiate the material W to bemeasured. The choice of radiation depends on the character and thedensity of the material W. Typical examples of beta emitters areStrontium 90 and Krypton 8S, and typical gamma emitters are Americium24l and Cesium 137. The shutter 48 and the associated mechanism may beconstructed according to the teachings of the copending application ofTrachevski et al. Ser. No. 634,363 filed Apr. 27, 1967.

The gauge of the present invention is particularly adapted for use inthe meaurement of the thickness or basis weight of sheet materialpassing from a rolling mill. The material W may accordingly be sheetaluminum which may be either hot or cold and which has a surface whichhas been bathed with oil at the vicinity of the rolling mill. Theresulting environment is one in which a mist or droplets of oil are entrained with the sheet and carried into the vicinity of the source anddetector heads. This oil mist has a tendency to accumulate and formdroplets of oil which, in a relatively short time, form on all of theexposed surfaces of the source and detector housings. This has noadverse affect per se on the gauge except when such droplets collect onthe surfaces of and in the vicinity of the windows 33 and 50, wheretheir presence results in an attenuation of the signal being received bythe detector 28. Further, as such droplets accumulate they reach thepoint where they drop by gravity, or run, or tom and collect in poolswhich will collect and run, thus forming an ever changing mass in thepass gap 21 and resulting in the introduction of an error in theread-out.

A further problem resides in the fact that the windows 32 and 50 are themost fragile exposed portion of the gauge. These windows are normallyformed of relatively thin sheet material such as sheet Mylar. They arethus subject to damage upon the occurrence of an industrial accidentwhich frequently consists of the breaking of the web W and the piling upof the broke against the gauge. Since the web is moving at a relativelyhigh velocity, which may be in the order of 500 feet per minute or more,considerable forces may momentarily result which are sufficient torupture or break a window if a portion of the web should come intocontact with the window.

The present invention is accordingly directed to structure whicheffectively purges the windows of the accumulation of dilatoriousmaterial, such as oil droplets, dust of other residue, and at the sametime provides mechanical protection for the windows. For this purpose,the detector housing l5 includes first plate means mounted thereon andindicated generally by the reference numeral 60 in FIG. 1. The plate 60is effectively positioned in the pass gap 21 and is mounted insuperimposed position on the lower surface of the detector housing sheet26, and is provided with an aperture 62 which is aligned substantiallywith the opening 30 and the window 33. The source housing 20, in turn,is provided with a second plate supported on the upper surface of thetop sheet 40, which second plate is similarly formed with an aperture 82aligned substantially with the window 50.

Referring first to the detector housing 15 and the arrangement of theplate 60, reference may be had to the views of FIGS. 5, 6 and 7. Thereit will be seen that the plate 60 is formed with a pair of upstandingside flanges 61 and 61' which are received over the adjacent side sheets23 and 24 of the housing 15 and may be retained by suitable screws 63 asshown in FIG. 1. The plate 60 is provided with an air inlet channel orpassage 64 which can best be seen in FIG. 8. The passage 64 may beformed by milling or otherwise cutting a slot in the upper surface ofthe plate 60. The passage 64 opens into a somewhat wider passage 65 ofthe same depth which, in turn, opens into one side of the plate aperture62. Further, the passage 65 extends from a region at the opposite sideof the aperture 62 across the remaining length of the plate 60 anddefines an air exit passage 68 through which air and entrained oilparticles may flow away from the measuring zone.

Since the window 33 is somewhat recessed in the housing 15 by reason ofit being positioned on the upper surface of the plate 26, as shown inFIG. 7, it is desirable to provide deflector means by which a majorportion of the air flow may be diverted upwardly into purging and wipingcontact with exposed lower surface of the window 33. For this purpose, atransversely oriented sheet metal deflector 70 may be positioned withinthe passage 65 adjacent the passage 64 and formed with an upwardlyextending tongue 72. The deflector 70 is mounted in spaced relation tothe floor of the passage 65 by reason of spaced washers 73 to provide asmall space 74 thereunder through which a portion of air may flow, forthe purpose of preventing the formation of a low pressure region orpocket on the lea side of the deflector 70 which would otherwise tend toaccumulate oil. The lower surface of the plate 60 is further formed witha circular and outwardly flared chamber 75 (FIG. 9) in the region of theaperture 62, leading away from the measuring region, along whichdroplets of oil may flow and effectively be removed from interferencewith the measurement.

Means for applying air under pressure to the inlet passage 64 includesan auxiliary end plate 76 mounted on the outside surface of the housingend sheet 27 and formed with a vertically aligned milled passageway 77on its inside surface. The lower end of the auxiliary plate 76 is formedwith a foot 78 which underlies the end of the sheet 27 and is inabutment with the adjacent edge of the plate 60 and is similarly formedwith a right-angled conduit or milled slot 79 communicating with theinlet passage 64.

Referring to FIGS. 2-4, it is seen that the plate 80 associated with thesource housing 20 is formed with a pair of depending flanges 83 and 84which extend downwardly adjacent to but spaced somewhat from the sideplates 42 and 43 and have inwardly turned lower ends 85. A pair of trackrails 88 are secured to the outer sides of the plates 42 and 43 adjacentthe upper surface of the top sheet 40 and provide means for supportingthe plate 80 as well as providing for longitudinal movement of theplate, for the purpose to be described.

The plate 80 is similarly formed with a milled slot defining an inletpassage 86 opening into the aperture 82. The plate is further formedwith a slot 87 defining an air exit passage diametrically opposite theinlet passage 86.

Means for applying air under pressure to the inlet passage 86 includes afurther auxiliary end plate 90. As best shown in FIG. 7, the upper endof the plate 90 is L-shaped and is brought over the upper surface of thesource housing end plate 45. The end plate 90 is formed with avertically aligned slot 92, and a right angle upper slot 93communicating with the inlet passage 86.

Means for applying air under substantial pressure for purging and wipingthe respective windows includes an air line 96 in the housing and aunion 97 in the end plate 27 for applying air to the passage 77 in theauxiliary end plate 76. The source housing may also include a conduit100 and a union 102 in the end plate 45 for applying air under pressureto the passage 86.

lt will be seen that the aperture 62 formed in the plate 60 is of adiameter which is somewhat less than that of the window 33, and theaperture 82 formed in the plate 80 is, again, somewhat less than that ofthe window 50. The respective plate apertures are preferably made assmall as possible without interferring with the path of radiationflowing from the source 47 to the detector 28, to provide additionalmechanical protection for the respective windows. As shown in H0. 2, theaperture 82 need not necessarily be circular, and a rectangular aperturemay be suitable or preferred.

In the event that the window 33 becomes damaged, it may be replaced byaccess through the removal of the upper sheet 22 of the housing 20.However, the source window 50 is removed by access through the pass gap22. Accordingly, the plate 80 associated with the housing 20 is movablelongitudinally of the lower arm 18 on the track rails 88. A pair ofdepending pins 105 are normally received over the ends of the rails, asshown in H6. 4, and serve to key the plate 80 in place in abutmentagainst the inwardly turned end of the auxiliary end plate 90, and whenit is desired to slide the plate 80 inwardly, it may be raised or liftedto the point where the pins 105 clear the end of the rails, the inwardlyturned ends 85 providing sufficient clearance for this purpose. Theplate 80 may then he slid inwardly to expose the window 50 as well asthe source assembly 46 for service and replacement as desired.

ln the operation of the invention, air under substantial pressure, inthe order of 60-80 psi, for example, is applied to the conduits 96 and100. A purging curtain of air is thus applied against each of theexposed surfaces of the windows 33 and 50 through the respective inletpassages 64 and 86. In the case of the plate 60, the deflector 70directs a major portion of the air upwardly into the recess defined bythe opening 30, and the exhaust air as well as the entrained oil dropsare carried away from the measuring zone through the exit passage 65. Asmaller portion of the air passes through the space 74 beneath thedeflector 70 and prevents what might otherwise be a low pressure pocket,and purges this region of accumulation of oil droplets. As shown in FIG.9, the oil droplets, due to surface tension, tend to adhere to adjacentsurfaces and the outwardly flared chamfer 75 forms a path along whichthese droplets may flow away from the measuring region.

The air applied by the conduit 100 is directed by the auxiliary plate 90to enter the inlet passage 86. This passage is of substantial width,corresponding generally to the width of the aperture 82, and forms anair wipe across the upper surface of the window 50. While both of thewindows will necessarily be wetted by the oil particles, the movement ofair across these windows is such as to prevent the formation oraccumulation of droplets which would adversely affect the measurement.

Again, the spent air and entrained particles are carried through theexit passage 86 out of the measuring zone.

In the event of breakage of the web, the plates 60 and providesubstantial additional mechanical protection for the respective windows.While it is still possible to incur window breakage, the likelihood ofbreakage has been substantially reduced. While the present inventiondiscloses window protecting and air purging arrangements which areformed as auxiliary attachments to the respective source and detectorhousings, it is within the scope of the invention to form these partsintegrally with such housings.

While the form of apparatus herein described constitutes a preferredembodiment of the present invention, it is to be understood that theinvention is not limited to this precise form of apparatus, and thatchanges may be made therein without departing from the scope of theinvention which is defined in the appended claims.

What is claimed is:

1. In a nucleonic gauge for measuring the basis weight or thickness ofmoving strip material in which a source housing is positioned at oneside of a pass gap and has a window opening at said gap and a detectorhousing is positioned at the opposite side of said pass gap and also hasa window opening at said gap for receiving radiation from said source,in an industrial environment in which oils or other substances have atendency to deposit or collect on the source and detector windows, theimprovement comprising first plate means mounted on said detectorhousing and having means defining an aperture aligned substantially withsaid detector window and providing mechanical protection for said windowagainst damage which may occur upon the breakage of said strip materialor other industrial accident, means in said first plate means definingan air inlet passage opening at said plate aperture, second plate meansmounted on said source housing and having an aperture therein insubstantial alignment with said source window providing mechanicalprotection against damage to said source window, means in said secondplate means defining an air inlet passage opening at the aperturethereof, and means for applying air under pressure to said plate inletpassages, said plate means directing said air for flow across saidsource and detector windows and into effective oil-wiping impingementthereon for maintaining said windows substantially free of accumulationof deposits.

2. The gauge of claim 1 further comprising means defining an outwardlyflared chamfer at the aperture of one of said plate means for providinga radially outwardly inclined path for the flow of oil droplets awayfrom the window protected by said one plate means.

3. The gauge of claim 1 in which said detector window is recessed insaid detector housing and further comprising a transversely orienteddeflector in the region of said first plate aperture positioned toreceive air from said first plate inlet passage and to deflect said airagainst said detector window.

4. The gauge of claim 3 further comprising means mounting said deflectorin spaced relation to said first plate means providing a space for airto flow between it and said first plate means to prevent the formationof a low pressure pocket on the lea side of said deflector which wouldotherwise tend to accumulate oil droplets.

S. The gauge of claim 1 in which said plate apertures are smaller indiameter than the associated housing windows.

6. In a nucleonic gauge for measuring the basis weight or thickness ofmoving strip material in which a source housing is positioned at oneside of a pass gap and has a window opening at said gap and a detectorhousing is positioned at the opposite side of said pass gap and also hasa window opening at said gap for receiving radiation from said source,for use in an environment in which oil droplets are entrained with thestrip material to be measured and which have a tendency to deposit onthe source and detector windows, the improvement comprising a firstplate mounted on said detector housing and having means defining anaperture aligned substantially with said detector window with saidwindow being recessed therein for providing protection for said window,means in said first plate means defining an air inlet passageterminating at said aperture and an exit passage looking therefrom, asecond plate mounted on said source housing and having an aperturetherein in substantial alignment with said source window, with saidsource window being recessed therein providing protection against damageto said source window, means in said second plate defining an air inletpassage opening into the aperture thereof and an air exit passageleading therefrom, means for applying air under pressure to said firstplate inlet passage for flow across said detector window into theassociated said exit passage, and means for applying air under pressureto said second plate inlet passage for wiping flow across said detectorwindow and into the associated said exit passage, for maintaining saidwindows substantially free of accumulation of oil droplets.

7. The gauge of claim 6 further comprising rail means mounted on saidsource housing and providing for movement of said plate with respect tosaid source housing for access to said source window.

8. The gauge of claim 6 further comprising means defining an outwardlyflared chamfer on said first plate in surrounding relation to said firstplate aperture providing an outwardly inclined path for the flow of oildroplets away from said detector window.

9. The gauge of claim 6 in which apertures formed in said plates aresmaller in diameter than that of the associated housing windows.

1. In a nucleonic gauge for measuring the basis weight or thickness ofmoving strip material in which a source housing is positioned at oneside of a pass gap and has a window opening at said gap and a detectorhousing is positioned at the opposite side of said pass gap and also hasa window opening at said gap for receiving radiation from said source,in an industrial environment in which oils or other substances have atendency to deposit or collect on the source and detector windows, theimprovement comprising first plate means mounted on said detectorhousing and having means defining an aperture aligned substantially withsaid detector window and providing mechanical protection for said windowagainst damage which may occur upon the breakage of said strip materialor other industrial accident, means in said first plate means dEfiningan air inlet passage opening at said plate aperture, second plate meansmounted on said source housing and having an aperture therein insubstantial alignment with said source window providing mechanicalprotection against damage to said source window, means in said secondplate means defining an air inlet passage opening at the aperturethereof, and means for applying air under pressure to said plate inletpassages, said plate means directing said air for flow across saidsource and detector windows and into effective oilwiping impingementthereon for maintaining said windows substantially free of accumulationof deposits.
 2. The gauge of claim 1 further comprising means definingan outwardly flared chamfer at the aperture of one of said plate meansfor providing a radially outwardly inclined path for the flow of oildroplets away from the window protected by said one plate means.
 3. Thegauge of claim 1 in which said detector window is recessed in saiddetector housing and further comprising a transversely orienteddeflector in the region of said first plate aperture positioned toreceive air from said first plate inlet passage and to deflect said airagainst said detector window.
 4. The gauge of claim 3 further comprisingmeans mounting said deflector in spaced relation to said first platemeans providing a space for air to flow between it and said first platemeans to prevent the formation of a low pressure pocket on the lea sideof said deflector which would otherwise tend to accumulate oil droplets.5. The gauge of claim 1 in which said plate apertures are smaller indiameter than the associated housing windows.
 6. In a nucleonic gaugefor measuring the basis weight or thickness of moving strip material inwhich a source housing is positioned at one side of a pass gap and has awindow opening at said gap and a detector housing is positioned at theopposite side of said pass gap and also has a window opening at said gapfor receiving radiation from said source, for use in an environment inwhich oil droplets are entrained with the strip material to be measuredand which have a tendency to deposit on the source and detector windows,the improvement comprising a first plate mounted on said detectorhousing and having means defining an aperture aligned substantially withsaid detector window with said window being recessed therein forproviding protection for said window, means in said first plate meansdefining an air inlet passage terminating at said aperture and an exitpassage looking therefrom, a second plate mounted on said source housingand having an aperture therein in substantial alignment with said sourcewindow, with said source window being recessed therein providingprotection against damage to said source window, means in said secondplate defining an air inlet passage opening into the aperture thereofand an air exit passage leading therefrom, means for applying air underpressure to said first plate inlet passage for flow across said detectorwindow into the associated said exit passage, and means for applying airunder pressure to said second plate inlet passage for wiping flow acrosssaid detector window and into the associated said exit passage, formaintaining said windows substantially free of accumulation of oildroplets.
 7. The gauge of claim 6 further comprising rail means mountedon said source housing and providing for movement of said plate withrespect to said source housing for access to said source window.
 8. Thegauge of claim 6 further comprising means defining an outwardly flaredchamfer on said first plate in surrounding relation to said first plateaperture providing an outwardly inclined path for the flow of oildroplets away from said detector window.
 9. The gauge of claim 6 inwhich apertures formed in said plates are smaller in diameter than thatof the associated housing windows.